Recently, display devices using an element which emits light at an intensity in accordance with a supplied electric current (hereinafter, such an element will be referred to as a “current light-emitting element”), for example, OLED display devices have been developed. Such a display device controls an amount of current supplied to the current light-emitting element by a driving transistor provided in each of pixels, and thus controls the gray scale of display.
Among such display devices, a type of OLED display device has a structure including a substrate on which a plurality of OLED elements and a plurality of driving transistors in correspondence with a plurality of pixels are formed (hereinafter, such a substrate will be referred to as an “array substrate”), and a counter substrate for transmitting light from the OLED elements. The counter substrate is bonded to the array substrate. The OLED elements each include a light-emitting (EL) layer. Above and below the light-emitting layer, an anode electrode and a cathode electrode for supplying a current for light emission are located. These electrodes are connected to a driver IC or an external power supply via a power supply line provided on the array substrate, and thus are supplied with a power supply voltage.
In order to minimize a transmission loss between the power supply and the electrodes, a type of conventional display device, for example, uses a thinner power supply line, uses a narrower power supply line, includes a plurality of power supply lines, or uses a power supply line formed of a plurality of layers, in an attempt to lower the resistance of the power supply line on the array substrate (see, for example, Patent Document 1: “Japanese Laid-Open Patent Publication No. 2001-154218”).
However, as the screen of the display device is enlarged, lines are extended; and as the display precision is raised and the frame area becomes narrower, an area where the lines can be located is made smaller. For these and other reasons, the resistance of the power supply line on the array substrate is raised. Therefore, in the conventional display device as described above, image quality may be undesirably deteriorated for the following reasons. A voltage drop makes the luminance insufficient; and since the voltage is different between an area proximal to the power supply and an area distal from the power supply, there occurs an in-plane luminance distribution (luminance gradient).
The present invention made in light of the above-described problems solves the problem that the resistance of the power supply line is raised to cause luminance gradient, which occurs when a display device having a large screen, a high precision and a narrow frame area is designed. Namely, the present invention has an object of providing an OLED display device including low-resistance power supply lines with no luminance gradient and with no restriction by the size of the area on the array substrate where the power supply lines can be located, and thus realize improved image quality.